Oracles are sometimes compared to a passive form sorcery. While sorcerers force things to happen, oracles understand what will happen.
While other diviners pose questions and seek answers, oracles are given answers in the form of omens, and must find the proper question.
Omens may come to oracles in the waking world, or while dreaming.
Oracles are both born and made. Some oracles seem to be born with the ability, others are given the gift as a sign of the grace of their god, and some few gain the ability gradually through their understanding of the world.
As people who display oracular ability are most often recruited intensely to the clergy, and some member of the clergy are counted among the most studious philosophers, the distinctions blur.
Born oracles often have a gift for sorcery as well. Or rather, sorcerers are more likely to be born with oracular ability than the rest of the population - even among them it is rare.
Sacral authorities often try to discredit secular oracles, and indeed most freelance oracles seem to be nothing more than charlatans.